1. Field of the Invention
The present invention relates to a method of increasing oil recovery by injecting an aqueous micellar fluid into a subterranean formation and, more particularly, to such a method which utilizes mixtures of mono-and disulfonates.
2. Setting of the Invention
After free flowing oil has been removed from an oil-bearing subterranean formation, a significant quantity of oil, as much as 50% of the original in-place oil, often times remains. This remaining oil is difficult to recover but certain tertiary or enhanced oil recovery (EOR) methods have proven to be effective. One method of tertiary recovery is the injection of fluids containing surfactants miscible with oil followed by a polymer thickened fluid into the reservoir to push or "sweep" residual or tertiary oil from an injection well to a production well. The micellar fluid is effective because of its ability to reduce or eliminate the oil retaining forces caused primarily by high interfacial tension (IFT). The polymer thickened fluid, usually called a mobility control fluid bank aids in maintaining the mobility and integrity of the micellar fluid in the reservoir.
The micellar fluid can be used alone or with a phase control fluid, containing a cosurfactant, usually a mixture of either one or two cosurfactants, such ethoxylated alcohols, to stabilize the micellar fluid and thereby increase its effectiveness and useful life as it moves through the reservoir. This type of micellar fluid system has been described in "Salinity and Hardness Tolerance Microemulsion Formations," Maini and Batycky, Petroleum Institute's Report RR-43, May, 1980.
The phase behavior of the surfactants and cosurfactants used has been extensively used to identify the optimum microemulsion formulation for the micellar fluids. The equilibration of the surfactant-oil-brine system has been shown to produce generally a lower-, upper-, and middle-phase microemulsion. It has been commonly thought to use the micellar systems that develop a middle phase microemulsion because minimal interfacial tensions (IFT) between both oil and brine with the surfactants can be obtained.
As beneficial as crude oil and/or vacuum gas oil sulfonate or other petroleum or synthetic sulfonate systems are in a micellar flooding operation, certain sulfonate components, such as the quasi-monosulfonates tend to stay together in a middle or upper phase microemulsion and the quasi disulfonates tend to stay in the water phase and this behavior can be detrimental to the micellar fluid performance. The inventor hereof knows of no reference which discloses or suggests the concept of varying the sulfonate mixture in the micellar fluid and/or the phase control fluid for optimum oil recovery. Further, the inventor hereof know of no reference which discloses or suggests utilizing a monosulfonate-rich mixture at the beginning of the introduction of the micellar fluid and/or utilizing a disulfonate-rich mixture at the trailing edge of the micellar fluid or in the phase control bank to overcome the above-described deficiencies.